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Storage pros: Big or small, you still have to hit the sweet spot

How does size matter in a hard drive?

Chris Evans - Independent storage consultant

The discussion on when 2.5-inch hard drives will usurp their ubiquitous 3.5-inch swarm of brothers has been raging for some time. The 3.5-inch form factor was a standard adopted many years ago in enterprise, mid-range and consumer devices. The use of 2.5-inch drives has obvious appeal in areas where power and space are an issue; laptops are a good example. We've also seen 2.5-inch drives implemented in rack-mounted servers, again for the space and power benefits. But why has adoption in enterprise storage arrays taken so long?

An argument put forward to me by EMC some years ago was the lack of dual supplier. EMC wanted to ensure drives were available from multiple sources in the event of quality or supply issues. This no longer holds water for two reasons: firstly, EMC happily provides customers with solid state drives from a sole supplier (STEC) and secondly, there are multiple suppliers of 2.5-inch drives in the market today. Perhaps there's an issue with the drive characteristics. This too is no longer a problem; 2.5-inch drives are as reliable as their 3.5-inch counterparts. They are as performant as 3.5-inch drives (both in throughput and interface speeds) and in fact they have other benefits in power, space and cooling.

A typical 2.5-inch 10K drive will require around 8 watts in typical operation, a 3.5inch drive around 10 watts. This may not seem like a huge saving, but when measured across a large array, provides significant savings. Power savings also implies cooling savings and this is additionally achieved on two fronts. First, for the same volume of space, more drives can be deployed, providing better airflow; second, the surface area to volume ratio is better for 2.5-inch drives (4.66 rather than 2.84), meaning a better heat dissipation characteristic.

So overall the 2.5-inch drive holds up well – until we start to discuss numbers of drives. For the same volume of space, more 2.5-inch drives can be deployed. This means more physical spindles across which to spread I/O and of course the counter-argument that more drives mean more failures. However this isn't necessarily a problem. Hard drives still continue to be configured in RAID groups and data loss is only an issue if two drives fail in the same RAID group. Where drive numbers do count is in the cost. Deploying more drives in an array will obviously cost more money and this increase in cost occurred because more 2.5-inch drives were required to deliver the same physical capacity as 3.5-inch drives in an array. We need to look at drive models available today to see that for 15K drives, 2.5-inch models still lag behind the 3.5-inch drives. However, the sweet spot for capacity is 10K, with both 2.5-inch and 3.5-inch models supporting up to 600GB. This means for the same capacity, 2.5-inch drives can be deployed in less space, require less power/cooling and provide the same levels of availability and performance.

I believe we are seeing the implementation of 2.5-inch drives by certain storage array vendors due to two factors: the cost/benefit ratio for 2.5-inch drives has been reached, and arrays are being implemented with advanced tiering technology that enables these drives to be used to the fullest extent. As a consequence, it is no longer necessary to purchase large numbers of faster (and high capacity) 15K drives, but a blended approach of SSD, SAS and SATA can provide effective performance at a lower cost.

I believe as the introduction of advanced tiering technology takes hold, we will see 3.5-inch drives relegated to providing mass, SATA-based storage, with 2.5-inch SSD and HDD drives delivering the bulk of I/O to the enterprise.

Chris M Evans is a founding director of Langton Blue Ltd.. He has over 22 years' experience in IT, mostly as an independent consultant to large organisations. Chris's blogged musings on storage and virtualisation can be found at www.thestoragearchitect.com.